Compressors in refrigeration systems raise the pressure of a refrigerant from an evaporator pressure to a condenser pressure. The evaporator pressure is sometimes referred to as the suction pressure and the condenser pressure is sometimes referred to as the discharge pressure. Many types of compressors, including screw compressors and scroll compressors, are used in such refrigeration systems. Such compressors are generally referred to herein as rotary compressors.
A rotary compressor includes a suction port and a discharge port that open into a working chamber of the compressor. For screw compressors, the working chamber includes a pair of meshed screw rotors that define a compression pocket between the screw rotors and interior walls of the working chamber. Refrigerant is received by the suction port and delivered to the compression pocket. Rotation of the rotors closes the compression pocket from the suction port and decreases the volume of the compression pocket as the rotors move the refrigerant toward the discharge port. Due to decreasing the volume of the compression pocket, the rotors deliver the refrigerant to the discharge port at a discharge pressure that is greater than the suction pressure.
For scroll compressors, the working chamber includes two spiral rotors interlocking with one another. One of the rotors is fixed and the other is driven by a motor to rotate relative to and against the sides of the other rotor. This generates suction, creating pockets of refrigerant that progress from the suction side around the rotors air and compress as they move toward the center of the rotors, where the refrigerant is then forced to the discharge port.
In addition to these general operational characteristics of rotary compressors in which the compression and movement of fluid produces axial thrust forces, rapid changes in rotational velocity of the rotors can cause axial rotor movement or thrust. If the rotors travel too far, they can contact surfaces of the compressor that were not intended to contact the rotors. As a result, the potential for failure and excessive wear of these components is created by axial movement of the rotors. While these thrust forces can be counter-acted by bearings, large thrust forces tend to cause wear of the thrust bearings, increase the loading on the compressor, and reduce reliability of the compressor and bearings over time. Therefore, further improvements in methods and systems for controlling axial thrust forces during operation of rotary compressors are desirable.